What’s with all the red lights?

Have you ever wondered why most modern machine vision systems use red light?

Yes, manufacturing engineers love the soft, romantic glow that comes from rows of red LEDS, but there’s more than aesthetics involved. I know of two reasons for ‘going red’:

Red LEDs are the least expensive, (I’m guessing this is driven by volume,) and no one likes to spend more than they have to.
In the early days of silicone-based CCD and CMOS sensors, or so I’m told, the material was most sensitive to photons with energies in the red part of the spectrum. So it made sense to match the wavelength of the light to the peak sensitivity of the detector.

But things change. CCD and CMOS sensor manufacturing is driven by consumer electronics, and consumers want digital cameras to produce images that agree closely with their eyes. So as the human eye has its’ peak sensitivity in the green part of the spectrum, (to better see the predator lurking in the bushes?) camera sensors are now engineered with the same response.

What does this mean for the Joe the machine vision engineer? Well, he should check the spectral response graph for the sensor in the camera(s) he’s going to use – most manufacturers put this on their web site – and consider using light of a wavelength the camera is best able to detect. (It’s a question of efficiency really – aim to ‘harvest’ the greatest amount of energy from the input.)

What this suggests, if you haven’t beaten me to the punchline, is that it may make sense to use green light rather than red.